The subject of this paper is the study of the problem of defining the crime of aggression in international law, primarily observing the historical development of the term from the very specific efforts in starting of its concretization, all the way to the current outcomes obtained with the mentioned development. Historically, creating the definition of the crime of aggression encountered many obstacles, mainly regarding the political occurrences of the time, which resulted with continued postponing of clear and precise determination of what the crime of aggression should encompass. The importance of several international documents is undisputed in order to completely understand the matter, especially the Charter of the United Nations and the Rome Statue of the International Criminal Court, whose relatively recent amendments are the consequence of a long-term effort of incorporating all the factors that make the crime of aggression. They represent success, taking into consideration the time period that had to pass and the path that had to be taken in order to reach the current situation of terminological unambiguousness and security. The importance of the Rome Statute of the International Criminal Court is also seen in the fact that it constituted the International Criminal Court, which as an institution has jurisdiction in deciding and represents the procedural establishment which implements the criminalisation of the crime of aggression. Taking into consideration how the crime of aggression is a defined term of a newer character, it is important to mention how its punishing, i.e. its procedural concretisation of criminalising is to be witnessed, along with observing the activity of the International Criminal Court within that context, and in accordance with the aforementioned, a more detailed and concrete analysis of successfulness of the existent definition of the crime of aggression follows.